Drive thru's are supposed to be for people on the go and in a hurry. I don't think that you should be able to make a purchase over a certain amount of money at a drive though.
My ex-boss was waiting in a drive thru line at Taco Bell one night for about a half hour around 3am. She was so pissed off that when she got up there she told them that she was ordering ONE of EVERYTHING on the menu. The night manager was pissed off...
She ended up dropping off most of the food off to those of us who were working third shift.
Thanks Pam!